The present invention generally concerns communications security and is more particularly directed to a scrambling circuit capable of providing satisfactory security in a high bit rate TDMA communications system.
In conventional TDMA systems where data security is desired, data encryption equipment has been placed at the data source with decryption equipment used at the receiver. For data rates of 64 kilobits or less, encryption is possible using commercial encryption equipment operating in a well known manner in accordance with the National Bureau of Standards (NBS) Data Encryption Standard (DES). However, even at such a relatively low bit rate, some situations involve a large number of 64 kilobit data streams each of which must be encrypted, thus requiring a large number of encryption devices and increasing system costs and complexity.
Until recently, data security in TDMA systems has not been a widespread concern and the few applications in which it has been necessary may have justified its expense and attendant performance limitations. However, with increasingly widespread use of TDMA communications, for example in the commercial marketplace where sensitive data such as bank transfer information and petroleum exploratory data is being transmitted via TDMA links, the need to provide data security is becoming increasingly important. Users of TDMA systems now desire to have the TDMA transmission bulk encrypted in accordance with the National Bureau of Standards DES, but for domestic TDMA systems this may necessitate a set of hardware to encrypt data at either 30 or 60 megabits. Such equipment is not readily available.
Rather than utilize encryption equipment, a measure of security can be provided by scrambling the TDMA transmission. A common scrambling technique is to generate a psuedo-random (PN) number sequence and to mix this sequence with the data bit stream, e.g. via an Exclusive OR gate. The scrambled data is then modulated and transmitted with energy that is substantially equally distributed across the bandwidth of the transmission channel. A descrambler utilizing an identical PN sequence generator is located at the receiver and utilizes an identical PN sequence to extract the information data. In order to accurately extract the information, however, the PN sequence generators at the data source and receiver must be operated in synchronism, and this is assured by periodically resetting both of the PN sequence generators to a known state. In typical TDMA systems, the PN sequence generators are loaded with an all ones pattern at the beginning of a sequence, with the sequence being started either at the beginning of a TDMA frame and running continuously for the duration of the frame or at the beginning of each burst and running for the duration of each burst.
While such scrambling techniques do provide a measure of security, the protection provided is not sufficient for sensitive information. An unauthorized user with the same receiver hardware can reset his own PN sequence generator to enable decryption of the data. Even if he does not know from the start the value to which the sequence generator should be reset each time, if the reset value is always the same he need only collect enough data to break the code once, after which he will have no trouble in thereafter receiving and descrambling all transmitted information.
A slight improvement in security is provided by a system such as disclosed in U.S. Pat. No. 3,659,046 to Angeleri et al. In that system, the PN sequence generators in the transmitter and receiver are concurrently reset in order to maintain synchronization, but the resetting is performed in response to a randomly gnerated pulse at the transmitter. Thus, an unauthorized user, even knowing the starting state to which the scrambler is to be reset, would have the additional problem of determining exactly at what times the resetting is to occur. Although this does provide an additional measure of security, the system would not be protected from an unauthorized user having the same receiver hardware. Such an unauthorized user would have his own PN sequence generator reset at the proper times by merely detecting the reset pulse.